Prices have been quite constant for a lot of years, but they more of less doubled in my part of the world since the war in Ukraine started, so I am glad that at least the cost of the EPs remained constant. There wasn't much difference between basegame of TS4 and last EP for TS3, yet basegme TS4 was barebones. I don't think it's a question of inflation, but maybe a case of maximizing profits by cutting costs. And there are also the kits, that for a limited number of objects bring in more revenue. If you view them as part of an EP, then there isn't much a price increase would realistically add to an EP.
